1. Technical Field of the Invention
The present invention relates to the operation and control of an amine-regeneration system, for instance, subsumed within a natural gas refinery plant. More specifically, the invention relates to a method for in situ monitoring of the concentration of acid gases, such as CO.sub.2 and H.sub.2 S, sorbed or complexed to the amine used to remove these acid-gases from natural gas. The method relies upon measuring the pH of the amine stream, whether rich or lean, from which acid-gas loading can be calculated.
2. Description of the Prior Art
Natural gas is a ubiquitous fuel having a variety of applications in both commercial and residential settings. Yet, before natural gas can be used for virtually any application, it must be refined. The most significant refining step is removal of waste or corrosive acid-gases, such as carbon dioxide (CO.sub.2), and hydrogen sulfide (H.sub.2 S). These gases are known in the industry as "acid-gases," because they lower the pH when dissolved in water. For example, hydrogen sulfide is highly toxic, even at trace levels, and therefore must be removed. Carbon dioxide reduces the heating value of natural gas, hence it must also be removed. The primary means by which acid-gases are removed from natural gas is by contact with a suitable base, which reacts with the acid(s) to form a salt. A preferred family of bases in the natural gas refining industry is a family of compounds known as alkanolamines. Alkanolamines are desirable because they react sufficiently well with the acid-gases, yet can be regenerated; that is, the acid-gases can be later removed in a separate step so that the same alkanolamine can be recycled through the system. Other parameters relevant to the selection of a compound for acid-gas removal from natural gas include: (1) loading (e.g., moles CO.sub.2 /moles reactant); (2) water solubility (the greater the solubility of the base, the more of it that can be used in the system; and (3) low corrosivity. Furthermore, an organic base, such as alkanolamine is preferred because, the organic portion of the amine has a greater affinity for the acid-gases (because of the greater polarity of the amine compared with the natural gas), hence the alkanolamine(s) adsorbs the acid-gases from the natural gas. Examples of typical alkanolamines used in the industry are monoethanol amine, diglycolamine, diethanolamine, diisopropanolamine, triethanolamine, and N-methyldiethanolamine. Finally, it should be noted that one recent trend in the industry is to select compounds which rather than "react" with the acid-gas, simply remove the acid-gases from the natural gas by relying upon the solvents' greater affinity for the acid-gases. Selexol and Rectisol are suitable exemplars of such physical solvents. "Mixed" solvents--i.e., solvents that rely both upon physical adsorption and chemical reaction to remove acid-gases--are also utilized.
Naturally, the capacity of a given quantity of amine to remove acid-gases from natural gas is limited. The quantity of acid-gas adsorbed onto a given quantity of amine is referred to as the "acid-gas loading," and represents the moles of acid-gas removed per mole of amine. In most systems currently practiced, the amine stream is continuously cycled through the system. Here, after loading, the amine must be regenerated--i.e., the acid-gases must be removed from amine (called "stripping"), so that the amine can be recirculated within the system. While the goal of the regeneration process is to remove completely the adsorbed acid-gases, the stripping process is incomplete, which means that the acid-gases are not completely removed from the amine. This, of course, reduces the efficiency of the amine (on a per-unit-of-amine basis) to remove acid-gases from the natural gas. Hence, in order to maximize the efficiency of the removal of acid-gases from the natural gas, the amine-regeneration process must be as effective as possible, i.e., it must return as much "lean" amine upstream where it contacts the natural gas, as possible. To accomplish this, the process variables that control acid-gas stripping from the amine stream must constantly be readjusted. Before this can be accomplished, the system operator must know the concentration of CO.sub.2 adsorbed by the amine (the "acid-gas loading") at various points in the system, particularly at the point at which the regenerated amine is returned upstream to react with the natural gas.
Most of the currently available methods for monitoring acid-gas loading are quite old--they involve traditional wet-chemistry techniques well known in the chemical art. Essentially, an aliquot of amine solution is obtained from the system. This aliquot is added to a solution of predetermined pH. This solution is then titrated until the pH returns to the predetermined pH. From the concentration and volume of the titrant needed to restore the pH, the concentrations of CO.sub.2 and H.sub.2 S, and their respective species, can be readily calculated.
This method is overwhelmed with difficulties. For one thing, it is expensive and time-consuming. A laboratory technician must collect the samples, and then perform the analyses in a laboratory. Besides the possibility of human error since the acid-gas loading must be determined manually (i.e., by titration), and besides the accumulation of hazardous wastes (e.g., potassium hydroxide and organic amines), which must be disposed of, there are other more significant reasons that these measurements may lack reliability. Measurements obtained by this way are static measurements, at best; they can be performed only periodically, or intermittently. This is highly problematic since a typical natural gas refining/amine-regeneration system is quite often not in equilibrium, due the constant addition of amine, water, etc. which causes "slug flow" in the system, or points of local disequilibria within the system. Hence what is needed is a method for the in situ monitoring of concentrations of acid gases, and which will allow continuous, real-time monitoring of acid-gas loading. Unfortunately, no such method exists to quickly and accurately measure dissolved gases in a dynamic system that would be suitable for use in a natural gas refining/amine-regeneration system. Dissolved gases are notoriously difficult to measure, particularly so in a system under pressure.
Hence the instant invention is directed towards such an in situ process for determining acid-gas loading by indirect measurement of another more easily determined solution parameter, which then acts as a proxy for acid-gas loading. Other systems are known in the art for determining some desired parameter by direct measurement of a different parameter. U.S. Pat. Nos. 5,208,164 and 5,162,084, issued to Cummings and Cummings et al., respectively, are directed to a process for determining the concentration of various anions (e.g., SCN.sup.-, Cl.sup.-, HCO.sub.2.sup.-) in an alkanolamine solution, by directly measuring electrical conductivity. Similarly, U.S. Pat. Nos. 5,196,345 and 4,273,146, issued to Cooper et al. and Johnson, respectively, measure pH and electrical conductivity to infer the concentration of acidic metal hydrides present in organic liquids, and the concentrations of various salts (e.g., NaCl), respectively. U.S. Pat. No. 4,323,092 issued to Zabel discloses a method to determine free chloride ion ("Cl.sup.- ") concentration by measuring total dissolved chlorine. U.S. Pat. Nos. 4,172,880 and 3,844,303, issued to Tzavos and Moon et al., respectively, disclose processes in which electrical conductivity is measured to infer total acidity and alkalinity, respectively. U.S. Pat. No. 4,277,343, issued to Paz discloses a method for determining alkalinity by measuring the partial pressure of CO.sub.2. U.S. Pat. No. 4,877,489, issued to Lloyd discloses a system for measuring electrical conductivity, and from these values the level of entrained bubbles is determined. Finally, U.S. Pat. No. 5,398,711, issued to Ardrey, Jr. discloses a general method for measuring various solution parameters such as conductivity or pH, and from those, inferring levels of the same or other solution components for the purpose of determining if a corrective response is needed.